In recent years, CMOS image sensors have emerged as the dominating alternative to charge coupled device (CCD) cameras.
CMOS active pixel sensor (APS) cameras have become a commodity, because of the low cost, low power, the ability to integrate analog and digital functions and the overall progress in reducing the readout noise and improving the overall performance CMOS active pixel sensor (APS) cameras store the integrated signal at each pixel as an amount of charge, which is proportional to the photo-current and the stored charge is converted by in-pixel amplifier operating as source follower into voltage.
Modern low intensity conflicts and acts of terrorism are stressing the need for military and law enforcement agencies to locate sporadic sources of hostile fire. The impact of gun detection systems on the battlefield depends not only on performance parameters of the single system but also on the abundance of the system among fighting forces. Silicon based sensors, in particular CMOS Image sensors, have revolutionized low cost imaging systems but to date have not been used for gun muzzle flash detection, due to performance limitations, and low signal to noise ratio (SNR) in the visible spectrum.
Single Avalanche Photo-Diode (SPAD) pixels are devices that can provide an internal gain with high-bandwidth characteristics. SPADs include a p-n junctions engineered to withstand high current densities when operated above breakdown. This is also known as Geiger mode operation. Geiger mode pixels with suitable readout structures for quenching and counting are theoretically capable of single photon detection and hence are referred to as single-photon avalanche diodes (SPADs). In Geiger mode, the SPAD is biased above its breakdown voltage (BV), and a single photo-electron theoretically initiates a self-propagating avalanche caused by the iterative multiplication of both the electrons and holes at high internal-electric Field strengths. This phenomenon is known as an avalanche breakdown.
In-pixel quenching and counting circuitry detects the presence of this avalanche current, and subsequently drops the bias below the BV. After quenching, the bias can then be raised again, above breakdown, awaiting the arrival of another single photoelectron event, thereby resetting the Geiger SPAD pixel. When operated in the Geiger mode, the SPAD can theoretically achieve single photon sensitivity with sub nanosecond timing characteristics.